The present invention relates to an improvement in a screw press, specifically to the configuration of the screws used in a twin screw press.
Screw presses have been used for centuries. Common applications are extracting juices and dewatering fibrous materials. Basically a screw press is a machine in which a helicoid screw rotates inside a cylindrical perforated screen. Entering material is subjected to gradually increasing pressure as it moves toward the exit end of the press, forcing the liquid phase to expel through the screen.
Compression is generally achieved in a screw press by a combination of three features: (a) the diameter of the shaft of the screw is increased, forcing material against the screen; (b) the pitch of the flights of the screw is reduced, compressing the material; and (c) back-pressure is created within the screen chamber by use of a cone (door, choke, plate, stopper or plug) located at the cake discharge end of the press.
A major innovation in screw press design was patented in 1900 by Valerius D. Anderson. His U.S. Pat. No. 647,354 describes how interruptions can be made in the flighting of the screw. This results in improved performance on slippery or slimy materials such as spent brewers"" grain, orange peel, fish, and rendering offal. The use of screw presses with interrupted flights rapidly came to dominate in these applications.
Flighting is the helicoid (corkscrew) surface that wraps around the shaft of a screw, somewhat like threads on a rod.
An improvement to the Anderson design was the addition of stationary resistor teeth. These are mounted, fixed to the frame of the machine, so that they protrude through the screen at the locations where the screw flighting is interrupted. The teeth are in the flow of material passing through the press. They can range in length from very short to reaching almost to the screw shaft. They cause stirring which both reduces co-rotation (slippage) tendency and puts wet material against the screen.
The l/d ratio is the ratio of screw diameter to screen length. Short l/d ratios, compared to those of continuous screw presses, are typical of interrupted flight machines.
A weakness of the single screw press is that, lacking positive displacement, excessive slippage can occur in the press. Thus difficult (slimy) materials can tend to co-rotate with the screw, resulting in a loss of both throughput and dewatering capacity. The amount of compression that can be applied to a material is limited by its tendency to slip, even in interrupted screw machines.
In recent decades another type of screw press has been found to offer stronger dewatering performance. This is a double (twin) screw press that features a pair of overlapping screws mounted side-by-side. Since the screws have continuous flighting, relatively positive displacement is created. This
semi-positive displacement, combined with optional features such as increasing shaft diameter, tightening pitch, and discharge back-pressure, result in a tighter squeezing press.
The principal disadvantage of the conventional twin screw press is that it requires (a) constant flow of (b) consistent material. If the solids consistency of the feed material decreases, this sloppy material will purge from the discharge of the press. A similarly bad situation occurs if the consistency increases: excessive dewatering occurs and the press jams or locks on a solid mass of cake. Screw and gearbox failures can result when this occurs.
Because of their heavy pressing action these continuous flight presses characteristically require high torque and rigidity, which results in large gearboxes, shafts, bearings, and flighting. These characteristics put conventional twin screw presses at a cost disadvantage.
In a continuous flight screw press there is almost no stirring action within the press. This is true even with twin screw designs. xe2x80x9cIn at three o""clock, out at three o""clock,xe2x80x9d is how they are described. A disadvantage of this is that high l/d ratios are required in order to allow enough time for liquid in the material against the screw shaft to migrate to the screen surface. A higher l/d ratio results in a longer, more costly machine than one with a low l/d ratio.
The object of the subject invention is to gain, in one machine, the respective advantages of interrupted flight screw press and twin screw presses. The invention combines the high capacity and low horsepower, relative to size, of the interrupted screw press, along with the strong, positive throughput and dewatering features of the twin screw press.
The subject invention uses twin overlapping interrupted flight screws with stationary resistor teeth mounted at the interruptions. The interruptions and teeth stir the material being pressed, which has the effect of both reducing co-rotation and placing fresh, wet material against the screen surface. Thorough dewatering is achieved with the low l/d ratios typical of interrupted screw presses.
The use of twin overlapping screws achieves a degree of positive displacement, resulting in tight squeezing and a great reduction in slippage. This improves dewatering of slimy materials. On the other end of the scale, there is enough give to the interrupted configuration that jamming is minimized. In addition it has been found that slicing action resulting from the interruptions in the screw flighting reduces the torque (horsepower) requirement for proper dewatering.